Towards forecasting phreatic eruptions:
Examples from Hakone volcano and some global equivalents

Editorial Board (December 2019)
Editor-in-Chief
Yasuo Ogawa, Tokyo Institute of Technology, Japan

Vice Editors-in-Chief
Masato Furuya, Hokkaido University, Japan
Solid Earth Science

Nozomu Nishitani, Nagoya University, Japan
Space Science

Editors
Valerio Acocella
Ryosuke Ando
Kimiyuki Asano
Nanan Balan
Paul Bedrosian
Phil Cummins
Hao Dong
Koji Fukuma
Shin-Chan Han
James Hickey
Thomas Hobiger
Hauke Hussmann
Takeo Ito
Art Jolly
Tomokazu Kobayashi
Attila Komjathy

Alexey Kuvshinov
Juanjo Ledo
Huixin Liu
Yohei Miyake
Mitsuhiro Nakagawa
Junichi Nakajima
Haruhisa Nakamichi
Yasuhito Narita
Takuya Nishimura
Takaaki Noguchi
Masahito Nose
Makiko Ohtake
Keiji Ohtsuki
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Duggirala Pallamraju
Severine Rosat

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Tatsuhiko Saito
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Sunny TamYoshiyuki Tanaka
Yanbin Wang
Yih-Min Wu
Chuang Xuan
Takuji Yamada
Mare Yamamoto
Tadashi Yamasaki
Tatsuhiro Yokoyama
Kazunori Yoshizawa

Hiroo Kanamori
Jun'ichiro Kawaguchi
Takafumi Matsui
Hitoshi Mizutani

Toshitaka Tsuda
Zhongliang Wu
Kiyoshi Yomogida

Advisory Board
Benjamin Fong Chao
Bernard Chouet
Yoshimori Honkura
Toshihiko Iyemori

Journal Scope
Earth, Planets and Space (EPS) is the official journal of the Society of Geomagnetism and Earth, Planetary
and Space Sciences, The Seismological Society of Japan, The Volcanological Society of Japan, The
Geodetic Society of Japan, and The Japanese Society for Planetary Sciences.
EPS is a peer-reviewed, open-access journal published under SpringerOpen. It is an international journal
covering scientific articles in the entire field of earth and planetary sciences, particularly geomagnetism,aeronomy, space science, seismology, volcanology, geodesy, and planetary science. EPS also welcomesarticles in new and interdisciplinary subjects, and technical reports on instrumentation and software.
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Earth, Planets and Space(2019) 71:91
Mannen et al. Earth, Planets and Space
https://doi.org/10.1186/s40623-019-1068-9

Open Access

PREFACESpecial issue “Towards forecasting phreatic
eruptions: Examples from Hakone volcano
and some global equivalents”
Kazutaka Mannen1* , Diana Roman2, Graham Leonard3, Stephanie Prejean4 and Mitsuhiro Nakagawa5

Introduction
Volcanic eruptions are among the most spectacular natural phenomena on Earth and incandescent magmatic
eruptions in particular have fascinated humans throughout history. Among the wide variety of volcanic eruptions, phreatic eruptions, which are non-magmatic andtypically small in size, have received far less attention
from researchers, and the number of research papers onphreatic eruptions has been far lower than those on magmatic eruptions. This does not mean phreatic eruption isa rare phenomenon—in fact, phreatic eruptions are common and may in fact outnumber magmatic eruptions.
A recent extensive review counted 116 phreatic eruptions of 36 volcanoes during the period from 1900 to2015 (approximately one per year on average) in Japan,
which has very detailed records of volcanic eruptions(Oikawa et al. 2018). However, few of these eruptions
had interested volcanologists probably because limited
ash dispersal and geophysical observations did not allow
detailed analysis. The tragic eruption at Mt. Ontake in2014, which killed 63 hikers, and the eruption (from an
unexpected eruption center) at Mt. Kusatsu-Shirane in2018, which killed a self-defense force personnel at drill
seemed to change the mindsets of the volcanologists inJapan. Although detailed analysis of the 2018 eruptionof Mt. Kusatsu-Shirane (Ogawa et al. 2018) is still ongoing, a special issue for the 2014 eruption of Mt. Ontake
was organized in this journal, and compiled observations
and models of preparatory process of this unexpected

eruption, which resulted largest number of victims afterWorld War II (Yamaoka et al. 2016).
The phreatic eruption of Hakone volcano, Japan, in2015 was accompanied by a variety of intense precursors
and mitigation measures, which included establishmentof a no-entry zone by the municipal office and raising of
the Volcano Alert Level by Japan Meteorological Agency.
In contrast to Kusatsu-Shirane and Ontake, Hakone volcano is an ‘urbanized’ volcano, which enables researchers to access this potential eruption center frequently andto deploy geophysical instruments connected to electricand communication grids. The 2015 eruption of Hakone
is thus an unprecedentedly well-monitored phreatic
eruption. This special issue compiles a variety of papers
that propose models of the hydro-magmatic system of
Hakone and some global equivalents based on various
observations.
Here, we review the contributions in the special
issue with related papers published in other journals.

Content of the special issue
Modeling of phreatic eruptions

A phreatic eruption is an eruption without any eruptionof juvenile magma. However, recent studies imply that
subsurface injection of magma or magmatic fluid cantrigger volcanic unrest and subsequent phreatic eruptions (e.g., Yamaoka et al. 2016). In this special issue, twocontributions modeled the sequence from an event ofmagma or magmatic fluid injection at depth to a phreatic
eruption through various precursory phenomena.
Stix and de Moor (2018) compiled recent studies ofphreatic eruptions and proposed two endmembers forphreatic systems. Type 1 systems are characterized by asealed hydrothermal system. Injection of magmatic fluids*Correspondence: mannen@onken.odawara.kanagawa.jp
1
Hot Springs Research Institute of Kanagawa Prefecture, 586 Iriuda,
Odawara, Kanagawa 250‑0031, Japan
Full list of author information is available at the end of the article© The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium,provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license,
and indicate if changes were made.

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Towards forecasting phreatic eruptions: Examples from Hakone volcano and some global equivalents
Mannen et al. Earth, Planets and Space(2019) 71:91

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controls volcanic unrest. In the model, the sealing zone
has some permeability during background periods; however, a few months before the onset of volcanic unrest,changes in permeability of the zone began to restrict gas
migration from the magma chamber to the hydrothermal system. The restriction allowed infiltration of atmospheric gases such as Ar and N2 into the hydrothermal
system. The increase in seismicity during volcanic unrest
was triggered by breakage of the sealing zone, resulting
in a transfer of confined pressure beneath the sealingzone into the hydrothermal system as indicated by theobserved increase of CO2/H2O ratios.
Ueda et al. (2018) compiled geophysical and geological observations of a volcanic island named Ioto, whichis also known as Iwo-jima for the fierce battle between
Japan and the United States during World War II. At
this volcano, phreatic eruptions with seismic precursors
occur during intermittent uplift. This type of phreatic
eruption is interpreted as a result of boiling of the hydrothermal reservoirs triggered by deep magma intrusion.
The hydrothermal fluid reaches the surface through fault
systems developed near the margin of hydrothermal reservoirs and these types of eruptions form craters along
the fault zone. In contrast, phreatic eruptions at Ioto that
occur without any precursors are small and similar to
geyser eruptions.

causes pressurization of the hydrothermal system andeventual rupture of the seal, generating ballistics, and fineash. For a Type 2 system, input of magmatic fluids into anear-surface hydrothermal system vaporizes liquid water,
promoting eruptions which emit wet ash, lahars, and/or
ballistics. Since injection of magmatic fluids and ruptureof hydrothermal seals are potentially seismogenic processes, they suggest that monitoring of broadband seismicity and gas ratios is useful approaches to forecasting
phreatic eruptions.
Mannen et al. (2018) compiled geological and geophysical observations during the 2015 phreatic eruption of
Hakone Volcano and its precursory unrest, and proposedthat magma replenishment of a 10-km-deep magma
chamber and subsequent pressurization of the hydrothermal system caused the volcanic unrest. The seismic
activity reached its climax more than a month before theeruption; however, a pressure increase in the hydrothermal system continued even after the eruption. Since theeruption was caused by sudden formation of an open
crack filled with hydrothermal fluid, short-term forecasting of a phreatic eruption was challenging.
Cap rock and sealing zone

A phreatic eruption is a phenomenon that releases
hydrothermal fluids confined beneath a volcano. Such aconfinement is seemingly attained by geological structures such as cap rock or sealing zone. To understandphreatic eruptions, the characterization of the propertiesof these structures, including their location, depth, andfunction in the context of the magma-hydrothermal system of the volcano is necessary. In the special issue, threecontributions focused on investigations of cap rock andsealing zones (Ueda et al. 2018; Yoshimura et al. 2018;
Ohba et al. 2019).
Yoshimura et al. (2018) conducted an audio-frequency
magnetotelluric (AMT) survey at 39 sites, covering thewhole of Hakone caldera before the eruption, and established a three-dimensional model of the resistivity structure of the volcano. The survey found a significant (< 10Ωm) bell-shaped conductor beneath the center of the
volcano. The apex of the bell-shaped conductor locatesnear the center of the volcano beneath the 2015 eruption vent. Beneath the conductor is a seismic zone which
had been active during the previous periods of volcanic
unrest. Since the previous study proposed that the seismic activity of the volcano had been triggered by pressure rise or fluid migration (Yukutake et al. 2011), thebell-shaped conductor is interpreted as the cap rock that
confines a hydrothermal system.
Ohba et al. (2019) compiled long-term gas observations
and developed a hydrothermal model of Hakone volcano
in which a sealing zone just above the magma chamber

Geological study

Phreatic eruptions are the smallest type of volcanic eruption in terms of erupted volume. Since the volume oferupted material is so small, the resulting deposits are
washed out swiftly and it is very difficult to reconstructan ancient phreatic eruption from study of the geologic
record. Thus, detailed description and analysis of an
eruption deposit soon after a phreatic eruption is criticalto understand the eruption mechanism and to establish
mitigation measures. For example, a detailed analysis oferupted material of the 2015 Hakone eruption implied
the very shallow source of the explosion (Yaguchi et al.
2019). This special issue has four contributions documenting geological investigations immediately following
phreatic eruptions (Geshi and Itoh 2018; Kataoka et al.
2018, 2019; Kilgour et al. 2019). In addition, Ikehata et al.
(2019) is a rare but valuable contribution discussing theformation mechanism of sulfur deposits, which is a common but generally understudied volcanic deposit.
Kilgour et al. (2019) undertook a comprehensive
surface investigation just after an eruption at White
Island, New Zealand. They deployed drones to swiftly
map the distribution of ballistic blocks. Based on theobserved distribution and a simulation code named Ballista (Tsunematsu et al. 2014), they estimated the initial
velocity of ballistics as 50–65 m/s, which is very low

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Tanaka et al. (2018) implemented a numerical simulation of a conduit system, through which hydrothermal
fluid reaches the surface, and monitored crater temperature and pressure distribution of the edifice interior after
changing the permeability of the conduit. The simulation
showed how crater temperature can decrease before aphreatic eruption. They also showed the potential of theirsimulation to understand mechanisms of phreatic eruption with observations of crater temperature and grounddeformation.

when compared to global equivalents. In addition, they
revealed that topography around the eruption center significantly affects the distributions of surge deposits andballistics.
Geshi and Itoh (2018) described a pyroclastic density current (PDC) formed during the 2015 phreatomagmatic eruption at Kuchinoerabu volcano in detail.
The PDC was generated by partial collapse of an eruption column, which rose up to 9 km above sea level, andflowed along a valley to reach the coast of the island
approximately 2.4 km away from the source crater. The
estimated temperature of the PDC at Kuchinoerabu during this phreatic eruption was between 100 and 240–
270 °C, which is less than that of typical magmatic PDC(> 300 °C). However, the PDC of Kuchinoerabu was stillenergetic and trees near the vent area were broken.
Kataoka et al. (2018) emphasizes that characteristics
of post-eruptive lahars that occur after a single eruptive
event can have significant differences depending on thetrigger. After the 2014 Ontake eruption, two types oflahars occurred within 7 months: a rain-triggered typeand a rain-on-snow type. The rain-triggered lahar, which
took place during a rain storm 8 days after the eruption,
deposited muddy and high-clay content sediment. In
contrast, a rain-on-snow-type lahar, which is caused byheavy rain and snow melting, formed a fines-depleted
sandy and gravelly deposit. Such a significant differencein post-eruptive lahars should be considered when implementing simulations and formulating mitigation plans.
Kataoka et al. (2019) monitored sediment transport
by rivers running from the eruption center of the 2014
Ontake eruption and revealed that the influence of volcanic disturbance on the catchment continued for at least
10 months after the eruption and an additional 9 months
until the end of the snowmelt season in 2016.

InSAR

InSAR (Interferometric Synthetic-Aperture Radar) presently plays a key role in volcano monitoring. During
the 2015 Hakone eruption and unrest, InSAR data werecritically important for planning mitigation measures(Mannen et al. 2018) and has to date resulted in threeprominent literature contributions related to InSAR, twoin this special issue (Doke et al. 2018; Kuraoka et al. 2018)
and one in another journal (Kobayashi et al. 2018).
Doke et al. (2018) detected the open crack that wasformed by the 2015 eruption by analysis of ground deformation observed by satellite InSAR. The eruption center
was formed at the northern end of the open crack. Since
old craters align on the ground surface just above theopen crack, the 2015 eruption was interpreted as a reactivation of a pre-existing crack that was formed by earlier eruptions. The detailed analysis of Doke et al. (2018)detected a sill-like deflation source beneath the crack,
which is considered the source of hydrothermal fluidthat formed the open crack. The pre-eruptive pressurization of the hydrothermal system of the volcano was also
detected as local uplift by the InSAR analysis.
Kuraoka et al. (2018) installed a ground-based InSAR(GBInSAR) 4 days before the 2015 eruption to monitor
the local uplift observed by satellite InSAR. Fortunately,
the 2015 eruption took place within the monitoringarea and ground deformation associated with the eruption was recorded with high sampling rate (< 10 min).
The 2015 eruption initiated crack intrusion at 7:32 andemission of highly pressurized fluid as inferred from
infrasonic analysis (Yukutake et al. 2018). Curiously, the
GBInSAR interferometry failed at the onset time andduring the subsequent 46 min; however, ground deformation after the crack formation was still monitored indetail, and this successful observation implies significant
potential of this technology for volcano monitoring andalerting.
Kobayashi et al. (2018) undertook a detailed analysis of InSAR and GNSS data and proposed two inflation sources: a deep spherical source at 4.5 km below
sea level (BSL), interpreted as a potential magma chamber, and a shallow source (at approximately 150 m below

Simulation and analogue experiments

Simulations can examine the influence of a simple key
mechanism on the subsequent complex behavior of the
volcanic system. In this special issue, two contributions
document simple-but-insightful simulation experiments(Noguchi et al. 2018; Tanaka et al. 2018).
Noguchi et al. (2018) focused on rootless eruptions
caused by lava flows entering inland water or water-richsediment. This type of eruption forms craters with significant explosion energy distal from the actual source vent;
however, forecasting of these occurrences has remained
difficult. Their unique analogue experiment using cooking ingredients such as syrup and baking powder showsa non-linear relationship between reaction efficiency andbaking soda/poured heated syrup (= water/lava) proportion which may stem from a Rayleigh–Taylor instability
occurring between lava and substrate water or sediment.

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Yukutake et al. (2018) extracted infrasound signals
emitted from the eruption center of the 2015 Hakone
eruption from noisy (wind) data using the record of aco-located seismometer (Ichihara et al. 2012). Due to thepoor visibility at the time of eruption, the exact timingof the eruption onset remains obscure and conventional
geological observations only detected a lahar, which initiated in the late morning as the initial emission (Mannen et al. 2018). However, the extracted infrasound signal
indicates an emission of highly pressurized fluid at 07:32
A.M. (JST), which is the timing of formation of the initial open crack that triggered the eruption (Honda et al.
2018).
Honda et al. (2018) observed a rapid tilt change at
Hakone Volcano, which started 10 s before 07:33 A.M.
(JST) and lasted for approximately 2 min. The tilt change,
which occurred long before the initiation of ash dispersal at approximately 12:30 P.M. (JST), was observed not
only by tiltmeters but also by broadband seismometers,
which were temporarily deployed around the eruption
center. The tilt change was considered to be caused by acrack intrusion, and analysis of the tilt record revealed
the parameters of the crack, such as its location, depth,
and opening. Since the opening of the crack is small(< 10 cm), it is highly unlikely that magma (as opposed tohydrothermal fluids) filled the crack.
Harada et al. (2018) monitored the rate of inflation of
Hakone volcano using GNSS, beginning when precursory
unrest started approximately 3 months before the 2015
eruption, and modeled the inflation sources. The deep
source at 6.5 km BSL, which is interpreted as a magma
chamber, first started inflating at the end of March, thenthe shallow open crack at approximately 800 m ASL
started inflating in mid-May. Interestingly, both sources
continued inflating even after the eruption on Jun. 29
and until early August. This observation implies thatthe phreatic eruption did not relieve the pressure in thehydrothermal system significantly.
Yukutake et al. (2017) estimated the source location of
the continuous volcanic tremor observed during the 2015
Hakone eruption using a cross-correlation analysis of
waveform envelopes. The source of the tremor is determined to be near the vent of the eruption. The amplitudeof the tremor increased coincident with the occurrenceof impulsive infrasonic waves and the largest amplitude
was observed at the end of the eruption. The analysis suggests that both seismic and infrasonic waves were generated when a gas slug bursts at the surface of the vent.

the surface) which caused very local surface deformation detected by InSAR. Similar deformation sources arealso deduced in papers in the special issue; however, theemphasis of Kobayashi et al. (2018) is on synchronization of the deformation rate changes of the two sources,
which implies smooth fluid migration from deep to shallow. Kobayashi et al. (2018) also showed that subtle inflation of the two sources initiated in late 2014, long before
the acceleration of inflation beginning in May 2015.
Signals emitted by phreatic eruption—location and source
analysisPhreatic eruptions emit various waveforms that are triggered by migration of hydrothermal fluid; thus, the location and depth of these signals are of primary importancefor understanding the mechanism of the eruption. In
addition, the interpretation and timing of seismic andacoustic events in the sequence of the eruption can help
constrain eruption process. This special issue collected
intriguing examples from White Island, Kawah Ijen, andHakone (Yukutake et al. 2017, 2018; Caudron et al. 2018;
Harada et al. 2018; Jolly et al. 2018; Walsh et al. 2019).
Walsh et al. (2019) located eruptive pulses emitted during a phreatic eruption that occurred at White Island
on 27 April 2018 by a joint analysis combining acoustic
and seismic data. The locations of the eruption vent wereinferred using an amplitude source location method, andthe depth of eruption pulses was inferred from volcanic
acoustic–seismic ratios. After error analysis, the eruption
sources are shown to conceivably come from a single ventwith the eruption pulses gradually increasing in strengthwith time.
Jolly et al. (2018) focused on very-long-period (VLP)
seismic events during the 2018 phreatic eruption ofWhite Island volcano. The VLP was located through analysis of waveform semblance and the volumetric sourceof the largest VLP event was obtained by seismic waveform inversion. The pre-eruptive VLP seems to be linked
to advection of gas from the VLP source location at themagmatic carapace, approximately 800 to 1000 m depth.
Caudron et al. (2018) carefully processed broadband
seismic signals of VLP events from two similar volcanoes:
Kawah Ijen (Indonesia) and White Island (New Zealand).
The phreatic eruptions of both volcanoes initiated witha VLP seismic event at shallow levels beneath the crater
region. The VLP events may be triggered by excitationof gas trapped behind a ductile magma carapace, followed by response of shallow hydrothermal system. Since
signals emitted by these processes are long period andcan be recorded only by broadband seismometers, theauthors emphasize the importance of deploying broadband seismometers near active volcanic centers.

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Conclusion
This special issue is a collection of the latest studies onphreatic eruptions from various aspects. We expect that
this timely special issue will help further advances inunderstanding of phreatic eruptions, towards the goal of
forecasting their occurrence.

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Kataoka KS, Matsumoto T, Saito T et al (2018) Lahar characteristics as a functionof triggering mechanism at a seasonally snow‑clad volcano: contrastinglahars following the 2014 phreatic eruption of Ontake Volcano, Japan.
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Mannen K, Yukutake Y, Kikugawa G et al (2018) Chronology of the 2015 erup‑tion of Hakone volcano, Japan—geological background, mechanism of
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Ueda H, Nagai M, Tanada T (2018) Phreatic eruptions and deformation of Ioto
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ets Space 70:38. https://doi.org/10.1186/s40623‑018‑0811‑yWalsh B, Procter J, Lokmer I et al (2019) Geophysical examination of the 27
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Yaguchi M, Ohba T, Sago M (2019) The nature and source of the volcanic
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Yamaoka K, Geshi N, Hashimoto T et al (2016) Special issue “the phreatic erup‑tion of Mt. Ontake volcano in 2014” the Phreatic Eruption of Mt. Ontake
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Yoshimura R, Ogawa Y, Yukutake Y et al (2018) Resistivity characterisation of
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3‑018‑0848‑yAbbreviations
GBInSAR: Ground‑Based Interferometric Synthetic‑Aperture Radar; GNSS:
Global Navigation Satellite System; InSAR: Interferometric Synthetic‑Aperture
Radar; JST: Japan Standard Time; PDC: pyroclastic density current; VLP: very
long period.
Acknowledgements
Not applicable.
Authors’ contributions
All authors read and approved the final manuscript.
Funding
Not applicable.
Availability of data and materials
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Hot Springs Research Institute of Kanagawa Prefecture, 586 Iriuda, Odawara,
Kanagawa 250‑0031, Japan. 2 Department of Terrestrial Magnetism, Carnegie
Institution for Science, 5241 Broad Branch Road, NW, Washington, DC
20015‑1305, USA. 3 GNS Science, 1 Fairway Drive, Lower Hutt, New Zealand.
4
USAID‑USGS Volcano Disaster Assistance Program, Volcano Science Center,
4230 University Avenue, Anchorage, AK 99508, USA. 5 Graduate School of Sci‑ence, Hokkaido University, Kita 8 Nishi 5, Kita, Sapporo, Hokkaido 060‑0808,
Japan.
Received: 31 July 2019 Accepted: 3 August 2019

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Ikehata K, Date M, Ishibashi J et al (2019) Solid sulfur spherules near fumarolesof Hakone volcano, Japan. Int J Earth Sci 108(1):347–356. https://doi.
org/10.1007/s00531‑018‑1657‑z

5

Towards forecasting phreatic eruptions: Examples from Hakone volcano and some global equivalents
Mannen et al. Earth, Planets and Space(2019) 71:91

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Yukutake Y, Ito H, Honda R et al (2011) Fluid‑induced swarm earthquake
sequence revealed by precisely determined hypocenters and focal mech‑anisms in the 2009 activity at Hakone volcano, Japan. J Geophys Res Solid
Earth 116:B04308. https://doi.org/10.1029/2010JB008036
Yukutake Y, Honda R, Harada M et al (2017) Analyzing the continuous volcanic
tremors detected during the 2015 phreatic eruption of the Hakone
volcano. Earth Planets Space 69:164. https://doi.org/10.1186/s4062
3‑017‑0751‑yYukutake Y, Ichihara M, Honda R (2018) Infrasonic wave accompanying a crack
opening during the 2015 Hakone eruption. Earth Planets Space 70:53.
https://doi.org/10.1186/s40623‑018‑0820‑x

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in pub‑lished maps and institutional affiliations.

6Earth, Planets and SpaceFRONTIER LETTER

Open Access

Understanding and forecasting phreatic eruptions driven
by magmatic degassing
John Stix* and J. Maarten de Moor
Earth, Planets and Space 2018, 70:83 DOI:10.1186/s40623-018-0855-z
Received: 21 December 2017, Accepted: 1 May 2018, Published: 18 May 2018

Abstract
This paper examines phreatic eruptions which are driven by inputs of magma and magmatic gas. We synthesize data from several
significant phreatic systems, including two in Costa Rica (Turrialba and Poás) which are currently highly active and hazardous. We
define two endmember types of phreatic eruptions, the first (type 1) in which a deeper hydrothermal system fed by magmatic gases
is sealed and produces overpressure sufficient to drive explosive eruptions, and the second (type 2) where magmatic gases aresupplied via open-vent degassing to a near-surface hydrothermal system, vaporizing liquid water which drives the phreatic eruptions.
The surficial source of type 2 eruptions is characteristic, while the source depth of type 1 eruptions is commonly greater. Hence, type1 eruptions tend to be more energetic than type 2 eruptions. The
first type of eruption we term “phreato-vulcanian”, and thesecond we term “phreato-surtseyan”. Some systems (e.g.,
Ruapehu, Poás) can produce both type 1 and type 2 eruptions,and all systems can undergo sealing at various timescales. We
examine a number of precursory signals which appear to beimportant in understanding and forecasting phreatic eruptions;these include very long period events, banded tremor, and gas
ratios, in particular H2S/SO2 and CO2/SO2. We propose that if
these datasets are carefully integrated during a monitoring
program, it may be possible to accurately forecast phreatic
eruptions.

Keywords: Phreatic eruptions, Magmatic inputs, Overpressure,
Sealing, Vaporization, Forecasting

Graphical abstract*Corresponding author: John Stix, stix@eps.mcgill.ca

FRONTIER LETTER

Open Access

Pyroclastic density currents associated with the 2015
phreatomagmatic eruption of the Kuchinoerabujima volcano
Nobuo Geshi* and Jun’ichi Itoh
Earth, Planets and Space 2018, 70:119 DOI:10.1186/s40623-018-0881-x
Received: 7 August 2017, Accepted: 22 June 2018, Published: 11 July 2018

Abstract
A pyroclastic density current (PDC) was observed with the phreatomagmatic eruption of the Kuchinoerabujima volcano, southern Japan,
on May 29, 2015. The PDC flowed down in all direction from the source crater, forming three major branches. The PDC reached thewestern coast of the island and the edge of the habitation area, which is ~ 2.4 km away from the source crater. The average speed of PDC
along the Mukaehama River was estimated as 42 m/s. The PDC involved both surge and block-and-ash flow. Pyroclastic surge left thinlayer of volcanic ash, though no remarkable signature of the lateral transportation was found in the surge deposit. The surge deposit
occupies ~ 85% of the area covered with the PDC. Block-and-ash flow deposits werealso recognized, but their distribution was limited mainly at the foot of the steep slopeof the edifice of Shindake. The erupted materials from the May 29, 2015 eruption were
estimated to be ~ 1.3 × 109 kg, including 2.4 × 108 kg of the PDC deposit and~ 1.1 × 109 kg of the fallout deposit in proximal and distal area. The PDC deposit
consisted of rock fragments with various degrees of hydrothermal alteration. The
least-altered glassy blocks of andesite, which was the candidate for the juvenile
materials, occupied less than 10% of the deposit. The area covered by the PDC wasdamaged by both mechanical and thermal effects. The dynamic pressure of the PDC
damaged the forest in the inner portion of the area covered with the PDC. The thermal
influence of the PDC caused dieback of the vegetation and deformation of some plastic
materials. The absence of carbonization of wood in PDC indicates that the temperature
was below the ignition temperature of wood. These observations suggest that thetemperature of the PDC was between 100 and 240–270 °C.

Keywords: Pyroclastic density current, Pyroclastic surge, Phreatomagmatic
eruption, Kuchinoerabujima volcano

Graphical abstract*Corresponding author: Nobuo Geshi, geshi-nob@aist.go.jp
7Towards forecasting phreatic eruptions: Examples from Hakone volcano and some global equivalents

FULL PAPER

Open AccessAnalyzing the continuous volcanic tremors detected during
the 2015 phreatic eruption of the Hakone volcano
Yohei Yukutake*, Ryou Honda, Masatake Harada, Ryosuke Doke, Tatsuhiko Saito, Tomotake Ueno,
Shin’ichi Sakai and Yuichi Morita
Earth, Planets and Space 2017, 69:164 DOI:10.1186/s40623-017-0751-y
Received: 12 July 2017, Accepted: 27 November 2017, Published: 6 December 2017

Abstract
In the present study, we analyze the seismic signals from a continuous volcanic tremor that occurred during a small phreatic eruption of the
Hakone volcano, in the Owakudani geothermal region of central Japan, on June 29, 2015. The signals were detected for 2 days, from June 29 toJuly 1, at stations near the vents. The frequency component of the volcanic tremors showed a broad peak within 1–6 Hz. The characteristics of thefrequency component did not vary with time and were independent of the amplitude of the tremor. The largest amplitude was observed at theend of the tremor activity, 2 days after the onset of the eruption. We estimated the location ofTilt changeaathe source using a cross-correlation analysis of waveform envelopes. The locations of volcanic
tremors are determined near the vents of eruption and the surface, with the area of the upper
extent of an open crack estimated using changes in the tilt. The duration-amplitude
distribution of the volcanic tremor was consistent with the exponential scaling law rather thanthe power law, suggesting a scale-bound source process. This result suggests that the volcanic
tremor originated from a similar physical process occurring practically in the same place. The
increment of the tremor amplitude was coincident with the occurrence of impulsive infrasonicwaves and vent formations. High-amplitude seismic phases were observed prior to thebbinfrasonic onsets. The time difference between the seismic and infrasonic onsets can be
explained assuming a common source located at the vent. This result suggests that bothseismic and infrasonic waves are generated when a gas slug bursts at that location. The
frequency components of the seismic phases observed just before the infrasonic onset weregenerally consistent with those of the tremor signals without infrasonic waves. The burst of agas slug at the surface vent may be a reasonable model for the generation mechanism of the
volcanic tremor and the occurrence of impulsive infrasonic signals.
20

Frequency [Hz]

18
16
14
12
10
8
6
4
2

03:00 07:00

11:00 15:00 19:00
June 29 2015

23:00

03:00

07:00

11:00 15:00
June 30 2015

19:00

23:00

03:00 07:00
July 01 2015log10[nm/s•s]

15-5

15-9

2

3

4

5

15-1

35
30

RMS [nm/s]

25
20
15

103

10
5

03:00 07:00

Keywords: Volcanic tremor, Phreatic eruption, Duration-amplitude distribution, Hakone

Infrasonic wave [Counts/min]

N=1,693104

0

11:00 15:00 19:00
June 29 2015

23:00

03:00

07:00

RMS at OWD (1–6 Hz)
Impulsive infrasonic wave (Counts/min)Timing of vent formation

11:00 15:00
June 30 2015

19:00

23:00

03:00 07:00
July 01 2015

Ash fall
Cumulative number of VT events

Fig. 3

volcano*Corresponding author: Yohei Yukutake, yukutake@onken.odawara.kanagawa.jp

FULL PAPER

Open AccessPhreatic eruptions and deformation of Ioto Island(Iwo-jima), Japan, triggered by deep magma injection
Hideki Ueda*, Masashi Nagai and Toshikazu Tanada
Earth, Planets and Space 2018, 70:38 DOI:10.1186/s40623-018-0811-y
Received: 18 December 2017, Accepted: 27 February 2018, Published: 12 March 2018

Abstract
On Ioto Island (Iwo-jima), 44 phreatic eruptions have been recorded since 1889, when people began to settle there. Four ofthese eruptions, after the beginning of continuous observation by seismometers in 1976, were accompanied by intense
seismic activity and rapid crustal deformation beforehand. Other eruptions on Ioto were without obvious crustal activities. In
this paper, we discuss the mechanisms of phreatic eruptions on Ioto. Regular geodetic surveys and continuous GNSS
observations show that Ioto intermittently uplifts at an abnormally high rate. All of the four eruptions accompanied by the
precursors took place during intermittent uplifts. The crustal deformation before and after one of these eruptions revealed
that a sill-like deformation source in the shallow part of Motoyama rapidly inflated before and deflated after the beginning of
the eruption. From the results of a seismic array and a borehole survey, it is estimated that there is a layer of lava at a depth ofabout 100–200 m, and there is a tuff layer about 200–500 m
beneath it. The eruptions accompanied by the precursors probably
occurred due to abrupt boiling of hot water in hydrothermal
reservoirs in the tuff layer, sealed by the lava layer and triggered byintermittent uplift. For the eruptions without precursors, thehydrothermal systems are weakly sealed by clay or probably
occurred on the same principle as a geyser because phreatic
eruptions had occurred beforehand and hydrostatic pressure isapplied to the hydrothermal reservoirs.

Keywords: Phreatic eruption, Caldera, Earthquake, Crustal
deformation, Precursor, Transient deformation

Graphical abstract*Corresponding author: Hideki Ueda, ueda@bosai.go.jp
8Earth, Planets and Space

FULL PAPER

Open Access

InSAR analysis for detecting the route of hydrothermal fluid
to the surface during the 2015 phreatic eruption of Hakone
Volcano, Japan
Ryosuke Doke*, Masatake Harada, Kazutaka Mannen, Kazuhiro Itadera and Jun Takenaka
Earth, Planets and Space 2018, 70:63 DOI:10.1186/s40623-018-0834-4
Received: 4 October 2017, Accepted: 5 April 2018, Published: 18 April 2018

Abstract
Although the 2015 Hakone Volcano eruption was a small-scale phreatic eruption with a discharged mass of only about 100 tons,
interferometric synthetic aperture radar successfully detected surface deformations related to the eruption. Inversion model of theunderground hydrothermal system based on measured ground displacements by ALOS-2/PALSAR-2 images showed that a crack
opened at an elevation of about 530–830 m, probably at the time of the eruption. A geomorphological analysis detected several old
NW–SE trending fissures, and the open crack was located just beneath one of the fissures. Thus, the crack that opened during the2015 eruption could have been a preexisting crack that formed during a more voluminous hydrothermal eruption. In addition, theinversion model implies that a sill deflation occurred at an elevation of about 225 m, probably at the time of the eruption. The
deflation of sill-like body represents a preexisting hydrothermal reservoir at an elevation of 100–
400 m, which intruded fluid in the open crack prior to eruption. The volume changes of the open
crack and the sill were calculated to be 1.14 × 105 m3 (inflation) and 0.49 × 105 m3 (deflation),
respectively. A very local swelling (about 200 m in diameter) was also detected at the eruption
center 2 months before the eruption. The local swelling, whose rate in satellite line-of-sight was
0.7–0.9 cm/day during May 2015 and declined in June, had been monitored until the time of theeruption, when its uplift halted. This was modeled as a point pressure source at an elevation of
about 900 m (at a depth of about 80–90 m from the ground surface) and is considered to be a minor
hydrothermal reservoir just beneath the fumarolic field. Our analysis shows that the northernmost
tip of the open crack reached within 200 m of the surface. Thus, it is reasonable to assume that thehydrothermal fluid in the open crack found a way to the surface and formed the eruption.

Keywords: Phreatic eruption, Hakone Volcano, InSAR, ALOS-2/PALSAR-2, Open crack,
Hydrothermal fluid

Fig. 10*Corresponding author: Ryosuke Doke, r-doke@onken.odawara.kanagawa.jp

FULL PAPER

Open Access

Chronology of the 2015 eruption of Hakone volcano, Japan:
geological background, mechanism of volcanic unrest anddisaster mitigation measures during the crisis
Kazutaka Mannen*, Yohei Yukutake, George Kikugawa, Masatake Harada, Kazuhiro Itadera
and Jun Takenaka
Earth, Planets and Space 2018, 70:68 DOI:10.1186/s40623-018-0844-2
Received: 10 November 2017, Accepted: 18 April 2018, Published: 26 April 2018

Abstract
The 2015 eruption of Hakone volcano was a very small phreatic eruption, with total erupted ash estimated to be in the orderof only 102 m3 and ballistic blocks reaching less than 30 m from the vent. Precursors, however, had been recognized at least 2 months before
the eruption and mitigation measures were taken by the local governments well in advance. In thispaper, the course of precursors, the eruption and the post-eruptive volcanic activity are reviewed,
and a preliminary model for the magma-hydrothermal process that caused the unrest and eruption
is proposed. Also, mitigation measures taken during the unrest and eruption are summarized anddiscussed. The first precursors observed were an inflation of the deep source and deep lowfrequency earthquakes in early April 2015; an earthquake swarm then started in late April. On May
3, steam wells in Owakudani, the largest fumarolic area on the volcano, started to blowout.
Seismicity reached its maximum in mid-May and gradually decreased; however, at 7:32 local timeon June 29, a shallow open crack was formed just beneath Owakudani as inferred from sudden tiltchange and InSAR analysis. The same day mud flows and/or debris flows likely started before 11:00
and ash emission began at about 12:30. The volcanic unrest and the eruption of 2015 can beinterpreted as a pressure increase in the hydrothermal system, which was triggered by magma
replenishment to a deep magma chamber. Such a pressure increase was also inferred from the2001 unrest and other minor unrests of Hakone volcano during the twenty-first century. In fact,
monitoring of repeated periods of unrest enabled alerting prior to the 2015 eruption. However,since open crack formation seems to occur haphazardly, eruption prediction remains impossible
and evacuation in the early phase of volcanic unrest is the only way to mitigate volcanic hazard.

Keywords: Hakone, Phreatic eruption, Lahar, Debris flow, Ash fall, Fumarole*Corresponding author: Kazutaka Mannen, mannen@onken.odawara.kanagawa.jp
9Graphical abstract

Towards forecasting phreatic eruptions: Examples from Hakone volcano and some global equivalents

FULL PAPER

Open AccessContention between supply of hydrothermal fluid andconduit obstruction: inferences from numerical simulations
Ryo Tanaka*, Takeshi Hashimoto, Nobuo Matsushima and Tsuneo Ishido
Earth, Planets and Space 2018, 70:72 DOI:10.1186/s40623-018-0840-6
Received: 29 December 2017, Accepted: 14 April 2018, Published: 4 May 2018

Abstract
We investigate a volcanic hydrothermal system using numerical simulations, focusing on change in crater temperature. Bothincreases and decreases in crater temperature have been observed before phreatic eruptions. We follow the system’s response
for up to a decade after hydrothermal fluid flux from the deep part of the system is increased and permeability is reduced at a
certain depth in a conduit. Our numerical simulations demonstrate that: (1) changes in crater temperature are controlled by
the magnitude of the increase in hydrothermal fluid flux and the degree of permeability reduction; (2) significant increases inhydrothermal flux with decreases in permeability induce substantial pressure changes in shallow depths in the edifice anddecreases in crater temperature; (3) the location of maximum pressure change differs between the mechanisms. The results ofthis study imply that it is difficult to predict eruptions by crater temperature change alone. One should be as wary of large
eruptions when crater
temperature decreases as
when crater temperature
increases. It is possible toclarify the implications ofchanges in crater
temperature withsimultaneous observationof ground deformation.
Graphical abstract*Corresponding author: Ryo Tanaka, ryo‑10@mail.sci.hokudai.ac.jp

FULL PAPER

Open Access

Lahar characteristics as a function of triggering mechanism
at a seasonally snow-clad volcano: contrasting lahars following
the 2014 phreatic eruption of Ontake Volcano, Japan
Kyoko S. Kataoka*, Takane Matsumoto, Takeshi Saito, Katsuhisa Kawashima, Yoshitaka Nagahashi,
Tsutomu Iyobe, Akihiko Sasaki and Keisuke Suzuki
Earth, Planets and Space 2018, 70:113 DOI:10.1186/s40623-018-0873-x
Received: 27 February 2018, Accepted: 12 June 2018, Published: 5 July 2018

Abstract
In association with the September 2014 phreatic eruption (VEI 1–2) at Ontake Volcano, a syn-eruptive and two post-eruptive lahars
occurred in the Akagawa–Nigorigawa River, southern flank of the volcano. The present contribution describes and discusses the contrasting
features of the two post-eruptive lahars, which caused a major impact on downstream river morphology, and re-examines the description of the
syn-eruptive lahar in the previous study. The first post-eruptive lahar occurred 8 days after the eruption by the rainstorm (October 5, 2014, before
the snowy season), and the second lahar was associated with the rain-on-snow (ROS) event on April 20, 2015, in the early spring of the snowmelt
season. The October rain-triggered lahar, which can be interpreted as a cohesive debris flow, reached at least ~ 11 km downstream and left muddy
matrix-rich sediments with high clay content (10–20 wt% of clay in matrix). The lahar deposits contain hydrothermally altered rock fragments,
sulfide/sulfate minerals, and clay minerals and show extremely high total sulfur content (10–14 wt%) in matrix part, indicating source material fromthe September phreatic eruption deposits. The presence of “rain-triggered” clay-rich lahar and deposits originating from a single small phreatic
eruption is important because usually such clay-rich lahars are known to occur in association with large-scale sector collapse and debris
avalanches. The April ROS-triggered lahar was caused by the heavy rain and accompanying snow melting. The lahar was dilute and partly erosionaland evolved into hyperconcentrated flow, which left fines-depleted sandy and gravelly deposits. Despite these lahars that originated from thesame volcanic source and occurring within a 7-month period, the flow and resultingaadepositional characteristics are totally different. These different types of lahars after asingle eruptive event need different simulations and mitigation of lahar hazards withtiming (season) of the lahar onset. In comparison with rainfall intensity, snow-melting rate,
and the contrasting lahars occurred in 2014/2015, it is postulated that the generation, size,
and types of lahars can vary with the timing of eruption, whether it happens during the
pre-snow season, snow season, or rainy season.

Keywords: Ontake Volcano, 2014 eruption, Phreatic eruption, Clay-rich lahar,
Rain-on-snow, Cohesive debris flowb*Corresponding author: Kyoko S. Kataoka, kataoka@gs.niigata-u.ac.jp
10cFig. 2

Earth, Planets and Space

FULL PAPER

Open AccessPrecursory tilt changes associated with a phreatic eruptionof the Hakone volcano and the corresponding source model
Ryou Honda*, Yohei Yukutake, Yuichi Morita, Shin’ichi Sakai, Kazuhiro Itadera and Kazuya Kokubo
Earth, Planets and Space 2018, 70:117 DOI:10.1186/s40623-018-0887-4
Received: 14 January 2018, Accepted: 27 June 2018, Published: 11 July 2018

Abstract

Tilt [

radian]Tilt [

radian]The 2015 unrest of the Hakone volcano in Japan, which began on April 26, generated earthquake swarms accompanied by long-term
deformation. The earthquake swarm activity reached its maximum in mid-May and gradually calmed down; however, it increased
again on the morning of June 29, 2015. Simultaneously with the earthquake increase, rapid tilt changes started 10 s before 07:33(JST) and they lasted for approximately 2 min. The rapid tilt changes likely reflected opening of a shallow crack that was formed near
the eruption center prior to the phreatic eruption on that day. In this study, we modeled the pressure source beneath the eruption
center based on static tilt changes determined using both tilt meters and broadband seismometers. In the best-fit model, the source
depth was 854 m above sea level, and its orientation (N316°E) agreed with the direction of maximum compression estimated basedon focal mechanism and S-wave splitting data. The extent of the crack
10
KZY NS
opening was estimated to be 4.6 cm, while the volume change was
8
KZY EW
5
3
6
approximately 1.6 × 10 m . The top of the crack reached to approximately
4
NNT NS
150 m below the eruption center. Because the crack was too thin to be
2
NNT EW0penetrated by magma, the crack opening was attributed to the intrusion-2of hydrothermal water. This intrusion of hydrothermal water may have
0
60
120
180
240
300
360
420
480
lapsed time after 7:32 on June 29 [sec]triggered the phreatic eruption. Reverse polarity motion with respect to
0.4
that expected from crack opening was recognized in 1 Hz tilt records
SSN NS
0.2
SSN EW
TNM NS
during the first 20 s of the intrusion of hydrothermal water. This motion,
0
TNM EW-0.2
not the subsidence of volcanic edifice, was responsible for the observed-0.4
KOM NS
displacement.
KOM EW-0.6-0.8

Keywords: Hakone volcano, Broadband seismogram, Tilt change,-1

Pressure source model

KZR NSKZR EW
0

60

120

180

240

300

360

420

480

lapsed time after 7:32 on June 29 [sec]Fig. 3*Corresponding author: Ryou Honda, ryou@onken.odawara.kanagawa.jp

FULL PAPER

Open Access

Anatomy of phreatic eruptions
Corentin Caudron*, Benoit Taisne, Jurgen Neuberg, Arthur D. Jolly, Bruce Christenson,
Thomas Lecocq, Suparjan, Devy Syahbana and Gede Suantika
Earth, Planets and Space 2018, 70:168 DOI:10.1186/s40623-018-0938-x
Received: 10 April 2018, Accepted: 6 October 2018, Published: 24 October 2018

Abstract
This study investigates phreatic eruptions at two similar volcanoes, Kawah Ijen (Indonesia) and White Island (New Zealand). By
carefully processing broadband seismic signals, we reveala seismic signatures and characteristics
c of these eruptions. At both
volcanoes, the phreatic eruptions are initiated by a very-long-period (VLP) seismic event located at shallow depths between
700 and 900 m below the crater region, and may be triggered by excitation of gas trapped behind a ductile magma carapace.
The shallow hydrothermal systems respond in different ways. At Kawah Ijen, the stress change induced by VLPs directly
triggers an eigenoscillation of the hyperacidic lake. This so-called seiche is characterized by long-lasting, long-period
oscillations with frequencies governed by the dimensions of the crater lake. A progressive lateral rupture of a seal below thecrater lake and/or fluids migrating toward the surface is seismically recorded ~ 15 min later as high-frequency bursts
superimposed to tilt signals. At White Island, the hydrothermal system later (~ 25 min) responds by radiating harmonic tremor
at a fixed location that could be generated througheddy-shedding. These seismic signals shed light ondbseveral aspects of phreatic eruptions, their generationand timeline. They are mostly recorded at periods
longer than tens of seconds further emphasizing theneed to deploy broadband seismic equipment closeto active volcanic activity.

Keywords: Volcanic monitoring, Phreatic eruption,
Volcanic lake, Broadband seismology

Fig. 7*Corresponding author: Corentin Caudron, Corentin.Caudron@gmail.com
11

Towards forecasting phreatic eruptions: Examples from Hakone volcano and some global equivalents

FULL PAPER

Open Access

Relating gas ascent to eruption triggering for the April 27,
2016, White Island (Whakaari), New Zealand eruption
sequence
Arthur Jolly*, Ivan Lokmer, Bruce Christenson and Johannes Thun
Earth, Planets and Space 2018, 70:177 DOI:10.1186/s40623-018-0948-8
Received: 10 April 2018, Accepted: 30 October 2018, Published: 13 November 2018

Abstract
The April 27, 2016 eruption sequence at White Island was comprised of 6 discrete eruptive events that occurred over a 35-min
period. Seismicity included three episodes of VLP activity: the first occurring ~ 2 h and a second occurring 10 min prior to thefirst eruption. A third larger VLP event occurred just prior to the fourth eruption. A VLP source depth of 800–1000 m below thevent is obtained from an analysis of the waveform semblance, and a
volumetric source is obtained from waveform inversion of the largest VLP
event. Lag times between VLP occurrence and eruption onsets provide an
opportunity to examine gas migration and stress transfer models as
potential triggers to the eruptive activity. Plausible lag times for a deepgas pulse to the surface are obtained by application of a TOUGH2
computational model which suggests propagation times of 0.25–1.9 m/s
and are informed by previously measured White Island rock porosities andpermeabilities. Results suggest that pre-eruption VLP may be plausibly
linked to advection of gas from the VLP source at a magmatic carapace
located ~ 800–1000 m depth. Alternatively, the large VLP that occurredjust prior to the fourth eruption may be linked to a quasi-dynamic or
quasi-static stress perturbation.

Keywords: VLP earthquake, Gas ascent velocity, Dynamic stress, Static
stress, TOUGH2 modeling

Graphical abstract*Corresponding author: Arthur Jolly, a.jolly@gns.cri.nz

FULL PAPER

Open Access

Geophysical examination of the 27 April 2016 Whakaari/White
Island, New Zealand, eruption and its implications forvent physiognomies and eruptive dynamics
Braden Walsh*, Jonathan Procter, Ivan Lokmer, Johannes Thun, Tony Hurst, Bruce Christenson
and Arthur Jolly
Earth, Planets and Space 2019, 71:25 DOI:10.1186/s40623-019-1003-0
Received: 3 April 2018, Accepted: 17 February 2019, Published: 28 February 2019

Abstract
At approximately 09:36 UTC on 27 April 2016, a phreatic eruption occurred on Whakaari Island (White Island) producing aneruption sequence that contained multiple eruptive pulses determined to have occurred over the first 30 min, with acontinuing tremor signal lasting ~ 2 h after the pulsing sequence. To investigate the eruption dynamics, we used a
combination of cross-correlation and coherence methods with acoustic data. To estimate locations for the eruptive pulses,
seismic data were collected and eruption vent locations were inferred through the use of an amplitude source location
method. We also investigated volcanic acoustic–seismic ratios for comparing inferredinitiation depths of each pulse. Initial results show vent locations for the eruptive pulses were
found to have possibly come from two separate locations only ~ 50 m apart. These resultscompare favorably with acoustic lag time analysis. After error analysis, eruption sources areshown to conceivably come from a single vent, and differences in vent locations may not be
constrained. Both vent location scenarios show that the eruption pulses gradually increase instrength with time, and that pulses 1, 3, 4, and 5 possibly came from a deeper source thanpulses 2 and 6. We show herein that the characteristics and locations of volcanic eruptions
can be better understood through joint analysis combining data from several data sources.

Keywords: Amplitude source location, Infrasound, Source migration, Volcanic
acoustic–seismic ratio, White Island*Corresponding author: Braden Walsh, B.Walsh@massey.ac.nz
12Graphical abstract

Earth, Planets and Space

FULL PAPER

Open AccessPhreatic eruption dynamics derived from deposit analysis:
a case study from a small, phreatic eruption from
Whakāri/White Island, New Zealand
Geoff Kilgour*, Stephanie Gates, Ben Kennedy, Aaron Farquhar, Ame McSporran and Cameron Asher
Earth, Planets and Space 2019, 71:36 DOI:10.1186/s40623-019-1008-8
RReceived: 12 June 2018, Accepted: 25 February 2019, Published: 1 April 2019

Abstract
On 27 April 2016, White Island erupted in a multi-pulse, phreatic event that lasted for ~ 40 min. Six, variably sized pulses
generated three ballistic ejections and at least one pyroclastic surge out of the inner crater and onto the main crater floor.
Deposit mapping of the pyroclastic surge and directed ballistic ejecta, combined with numerical modelling, is used toconstrain the volume of the ejecta and the energetics of the eruption. Vent locations and directionality of the eruption are
constrained by the ballistic modelling, suggesting that the vent/s were angled towards the east. Using these data, a model isdeveloped that comports with the field and geophysical data. One of the main factors modifying the dispersal of the eruption
deposits is the inner crater wall, which is ~ 20 m high. Thiswall prevents some of the pyroclastic surge and ballistic
ejecta from being deposited onto the main crater floor but
also promotes significant inflation of the surge, generating
a semi-buoyant plume that deposits ash high on the crater
walls. While the eruption is small volume, the complexity
determined from the deposits provides a case study withwhich to assess the relatively frequent hazards posed by
active volcanoes that host hydrothermal systems. The
deposits of this and similar eruptions are readily eroded,
and for complete understanding of volcanic hazards, it isnecessary to make observations and collect samples soonafter these events.
Graphical abstract*Corresponding author: Geoff Kilgour, g.kilgour@gns.cri.nz

FULL PAPER

Open AccessTime variations in the chemical and isotopic compositionof fumarolic gases at Hakone volcano, Honshu Island, Japan,
over the earthquake swarm and eruption in 2015, interpreted
by magma sealing modelTakeshi Ohba*, Muga Yaguchi, Kana Nishino, Nozomi Numanami, Yasushi Daita, Chiho Sukigara,
Masanori Ito and Urumu Tsunogai
Earth, Planets and Space 2019, 71:48 DOI:10.1186/s40623-019-1027-5
Received: 7 April 2018, Accepted: 15 April 2019, Published: 25 April 2019

Abstract
Definite increases in the components ratios of CO2/H2O, CO2/H2S, CO2/CH4 and He/CH4 wereobserved at the fumarolic gases from Owakudani geothermal area located at the center of
Hakone volcanic caldera (Honshu Island, Japan), synchronized with the earthquake swarm in2015. Such variations were due to the dominance of a magmatic component over ahydrothermal component, suggesting the earthquake swarm was produced by the injection ofmagmatic gases into the hydrothermal system. The CO2/H2O ratio of magmatic gas wasestimated to be 0.0045 before the earthquake swarm, which increased up to 0.013 during theearthquake swarm, likely produced by the pressurization of magma as a result of magma sealing
where the pressure increment in magma was estimated to be 3% to the lithostatic pressure. The
H2O and CO2 concentration in magma were estimated to be 6.3 wt% and 20 wt ppm,
respectively, assuming a temperature 900 °C and a rhyolitic composition. In May 2015, a few
months prior to the earthquake swarm in May 2015, a sharp increase in the Ar/CO2 and N2/He
ratios and a decrease in the isotopic ratio of H2O were observed at the fumarolic gas. The
invasion of air into the hydrothermal system increased the Ar/CO2 and N2/He ratios. The decreasein the isotopic ratio of H2O was induced by partial condensation of H2O vapor.

a

b

c

d

e

Keywords: Active volcano, Fumarolic gas, Earthquake swarm, Phreatic eruption, Mt. Hakone,
Magma degassing, Magma sealing

Fig. 2*Corresponding author: Takeshi Ohba, takeshi_ohba@tokai‑u.jp
13

Towards forecasting phreatic eruptions: Examples from Hakone volcano and some global equivalents

EXPRESS LETTER

Open Access

Infrasonic wave accompanying a crack opening during
the 2015 Hakone eruption
Yohei Yukutake*, Mie Ichihara and Ryou Honda
Earth, Planets and Space 2018, 70:53 DOI:10.1186/s40623-018-0820-x
Received: 27 December 2017, Accepted: 21 March 2018, Published: 28 March 2018

Abstract
To understand the initial process of the phreatic eruption of the Hakone volcano from June 29 to July 01, 2015, we analyzed infrasounddata using the cross-correlation between infrasound and vertical ground velocity and compared the results of our analysis to the crustal
deformation detected by tiltmeters and broadband seismometers. An infrasound signal and vertical ground motion due to an
infrasound wave coupled to the ground were detected simultaneously with the opening of a crack source beneath the Owakudani
geothermal region during the 2-min time period after 07:32 JST on June 29, 2015 (JST = UTC + 8 h). Given that the upper end of the open
crack was approximately 150 m beneath the surface, the time for the direct emission of highly pressurized fluid from the upper end of
the open crack to the surface should have exceeded the duration of the inflation owing to the hydraulic diffusivity in the porous media.
Therefore, the infrasound signal coincident with the opening of the crack may reflect a sudden emission of volcanic gas resulting fromthe rapid vaporization of pre-existing groundwater beneath Owakudani because of the transfer of the volumetric strain change from thedeformation source. We also noticed a correlation pattern corresponding to discrete impulsive infrasound signals during vent formation,
which occurred several hours to 2 days after the opening of the crack. In
a 7:32:30 June 29, 2015b 7:32:50 June 29, 2015
particular, we noted that the sudden emission of vapor coincided with
Abrupt gas emission
Owakudani
accompanied
byTilt change
Tilt meterthe inflation of the shallow pressure source, whereas the eruptive burst
fumarolic activity
infrasonic wave
1000 mevents accompanied by the largest vent formation were delayed byVaporisation
approximately 2 days. Furthermore, we demonstrated that the
850 m
Groundwater nearboiling temperaturecorrelation method is a useful tool in detecting small infrasound signals
μ strain change
Crack opening
Crackand provides important information regarding the initial processes ofthe eruption.
VT event

Keywords: Infrasound signal, Vertical ground motion, Correlation,
Monitoring, Volcanic activity, Phreatic eruption

0 m (SL)

Increment of
fluid pressure

Fig. 5*Corresponding author: Yohei Yukutake, yukutake@onken.odawara.kanagawa.jp

EXPRESS LETTER

Open AccessResistivity characterisation of Hakone volcano, Central Japan,
by three-dimensional magnetotelluric inversion
Ryokei Yoshimura*, Yasuo Ogawa, Yohei Yukutake, Wataru Kanda, Shogo Komori, Hideaki Hase,
Tada-nori Goto, Ryou Honda, Masatake Harada, Tomoya Yamazaki, Masato Kamo, Shingo Kawasaki,
Tetsuya Higa, Takeshi Suzuki, Yojiro Yasuda, Masanori Tani and Yoshiya Usui
Earth, Planets and Space 2018, 70:66 DOI:10.1186/s40623-018-0848-y
Received: 29 December 2017, Accepted: 19 April 2018, Published: 26 April 2018

Abstract
On 29 June 2015, a small phreatic eruption occurred at Hakone volcano, Central Japan, forming several vents in the Owakudani geothermalarea on the northern slope of the central cones. Intense earthquake swarm activity and geodetic signals corresponding to the 2015 eruption
were also observed within the Hakone caldera. To complement these observations and to characterise the shallow resistivity structure of
Hakone caldera, we carried out a three-dimensional inversion of magnetotelluric measurement
data acquired at 64 sites across the region. We utilised an unstructured tetrahedral mesh forthe inversion code of the edge-based finite element method to account for the steep
topography of the region during the inversion process. The main features of the best-fit
three-dimensional model are a bell-shaped conductor, the bottom of which shows good
agreement with the upper limit of seismicity, beneath the central cones and the Owakudani
geothermal area, and several buried bowl-shaped conductive zones beneath the Gora andKojiri areas. We infer that the main bell-shaped conductor represents a hydrothermally alteredzone that acts as a cap or seal to resist the upwelling of volcanic fluids. Enhanced volcanic
activity may cause volcanic fluids to pass through the resistive body surrounded by the alteredzone and thus promote brittle failure within the resistive body. The overlapping locations of
the bowl-shaped conductors, the buried caldera structures and the presence of sodiumchloride-rich hot springs indicate that the conductors represent porous media saturated byhigh-salinity hot spring waters. The linear clusters of earthquake swarms beneath the Kojiri area
may indicate several weak zones that formed due to these structural contrasts.

Keywords: Hakone volcano, Magnetotellurics, Resistivity structure, Three-dimensional
inversion

Graphical abstract*Corresponding author: Ryokei Yoshimura, ryokei@eqh.dpri.kyoto‑u.ac.jp
14Earth, Planets and Space

EXPRESS LETTER

Open Access

Temporal changes in inflation sources during the 2015
unrest and eruption of Hakone volcano, Japan
Masatake Harada*, Ryosuke Doke, Kazutaka Mannen, Kazuhiro Itadera and Mikio Satomura
Earth, Planets and Space 2018, 70:152 DOI:10.1186/s40623-018-0923-4
Received: 28 March 2018, Accepted: 14 September 2018, Published: 21 September 2018

Abstract
Global navigation satellite system data from Hakone volcano, central Japan, together with GEONET data from the Geospatial Information
Authority of Japan, were used to investigate the processes associated with the volcanic activity in 2015, which culminated in a small phreatic
eruption in late June 2015. Three deep and shallow sources, namely spherical, open crack, and sill, were employed to elucidate the volcanic
processes using the observed GNSS displacements, and the MaGCAP-V software was used to estimate the volumetric changes of thesesources. Our detailed analysis shows that a deep inflation source at 6.5 km below
sea level started to inflate in late March 2015 at a rate of ~ 9.3 × 104 m3/day untilamid-June. The inflation rate then slowed to ~ 2.1 × 104 m3/day and ceased at theend of August 2015. A shallow open crack at 0.8 km above sea level started toinflate in May 2015 at a rate of 1.7 × 103 m3/day. There was no significant volumetricchange in the shallow sill source during the volcanic unrest, which is evident frominterferometric synthetic aperture radar analysis. The inflation of the deep source
continued even after the eruption without a significant slowdown in inflation rate.
The inflation stopped in August 2015, approximately 1 month after the eruption
ceased. This observation implies that the transportation of magmatic fluid to a
deep inflation source (6.5 km) triggered the 2015 unrest. The magmatic fluid may
have then migrated from the deep source to the shallow open crack. The phreatic
eruption was then caused by the formation of a crack that extended to the surface.
However, steam emissions from the vent area during and after the eruption werebapparently insufficient to mitigate the internal pressure of the shallow open crack.
3

4

5

6

2015
7
8

9

10

11

12

1

2

3

2016
4

5

6

deep spherical inflation source

8.0E+06

8.0E+05

7.0E+06

7.0E+05

6.0E+06

6.0E+05

5.0E+06

5.0E+05

4.0E+06

4.0E+05
3.0E+05

3.0E+06

shallow open crack source

2.0E+06

2.0E+05
1.0E+05

1.0E+06

0.0E+00

0.0E+00

shallow sill source−1.0E+05−1.0E+06

eruption, Hakone

Number of EQ. (/day)−2.0E+06

Keywords: GNSS, Magma chamber, Hydrothermal system, Open crack, Phreatic

1.0E+06
9.0E+05

9.0E+06

1200
1000
800
600
400
200
0

Amount of volume change ( shallow dike and sill source, m3 )Amount of volume change ( deep spherical inflation source, m3 )1.0E+07

3

4

5

6

7
8
2015

9

3

4

5

6

7
8
2015

9

10

11

12

1

2

3

4
2016

5

6

10

11

12

1

2

3

4
2016

5

6

−2.0E+05

Fig. 4*Corresponding author: Masatake Harada, harada@onken.odawara.kanagawa.jp

EXPRESS LETTER

Open Access

Monitoring ground deformation of eruption center byground-based interferometric synthetic aperture radar(GB-InSAR): a case study during the 2015 phreatic eruptionof Hakone volcano
Senro Kuraoka*, Yuichi Nakashima, Ryosuke Doke and Kazutaka Mannen
Earth, Planets and Space 2018, 70:181 DOI:10.1186/s40623-018-0951-0
Received: 6 April 2018, Accepted: 8 November 2018, Published: 20 November 2018

Abstract
We successfully monitored the ground deformation of an eruption center during the 2015 phreatic eruption of Hakone volcano,Japan, using ground-based interferometric synthetic aperture radar (GB-InSAR). GB-InSAR has been developed and applied over the
past two decades and enables the frequent (< 10 min) aerial monitoring of surficial deformation of structures and slopes. We installeda GB-InSAR 4 days before the eruption of Hakone volcano on June 29, 2015, and monitored the ground deformation of an area where
uplift was detected by a satellite InSAR. The ground deformation observed by the GB-InSAR began suddenly on the morning of June
29 almost coincident with the intrusion of hydrothermal fluid that wasinferred by other geophysical observations. The hydrothermal crack is
considered to have caused the eruption, which was known by an ash fall 5 hlater. The GB-InSAR results indicated a significant uplifted area which isapproximately 100 m in diameter, and new craters and fumaroles werecreated by the eruption in and around the area. The displacement reached
up to a total of 45 mm until the evening of June 29 and continued at least
until the morning of July 1. During our observation, the displacement rate
decreased twice, and the timing of each decrease seemed to correspond to
the formation of new conduits as implied from geophysical observations.

Keywords: GB-InSAR, Hakone volcano, Ground deformation, Phreatic
eruption, Hydrothermal fluid*Corresponding author: Senro Kuraoka, a4982@n-koei.co.jp
15

Fig. 4

Towards forecasting phreatic eruptions: Examples from Hakone volcano and some global equivalents

EXPRESS LETTER

Open AccessExperimental approach to rootless eruptions using kitchenmaterials
Rina Noguchi*, Ai Hamada, Ayako I. Suzuki and Kei Kurita
Earth, Planets and Space 2018, 70:208 DOI:10.1186/s40623-018-0974-6
Received: 10 April 2018, Accepted: 10 December 2018, Published: 27 December 2018

Abstract
Rootless eruption is caused by lava flow entering inland water or wet sediment and forms craters and cones far from theactual vent. Since the rootless eruption can be significantly explosive, the possibility of rootless eruption is critical for hazard
management; however, forecasting occurrence and explosiveness of a rootless eruption remains difficult because
explosiveness is considered to correlate nonlinearly with the mass ratio of hot lava and water. Here, we show the nonlinear
nature of this type of eruption roots from the nonlinearly determined area between lava and underlying wet sediment as afunction of water content within the wet sediment by a series of analog experiments can be implemented in a kitchen. In ouranalog experiment, lava and wet sediment were replaced by heated syrup and mixture of baking soda (sodium bicarbonate)
and cake syrup. From measurements of mass loss due to CO2 gas emission during the heat-induced decomposition of baking
soda, we estimated the reaction efficiency for various proportions of baking soda and cake syrup in the substrate. We
observed a nonlinear dependence of CO2 emission; the peak efficiency was achieved for the substrate with 15 and 35 g ofbaking soda and cake syrup, respectively. Considering physical properties such as density and viscosity, we found that theRayleigh–Taylor instability between the poured heatedsyrup and the substrate can explain the observed nonlinear
dependence of CO2 emission. For natural settings, theresults of this study suggest that both the availability ofwater and rheological properties of the substrate affect theoccurrence and stability of hydrovolcanic eruptions.

Keywords: Rootless eruption, Lava–water interaction,
Kitchen earth experiment, Syrup, Soda

Graphical abstract*Corresponding author: Rina Noguchi, r-noguchi@ksvo.titech.ac.jp; r-noguchi@planeta.sci.isas.jaxa.jp

EXPRESS LETTER

Open Access

Suspended sediment transport diversity in river catchments
following the 2014 phreatic eruption at Ontake Volcano,
Japan
(a)

(b)

Kyoko S. Kataoka*, Takane Matsumoto, Takeshi Saito, Yoshitaka Nagahashi and Tsutomu Iyobe
Earth, Planets and Space 2019, 71:15 DOI:10.1186/s40623-019-0994-x
Received: 31 March 2018, Accepted: 24 January 2019, Published: 7 February 2019

Abstract
The present contribution investigates the temporal changes in volcaniclastic sediment transport over the 2-year period after the 2014 eruption of
Ontake Volcano in two small drainage basins where increased turbidity was observed immediately after the eruption. Two similar-sized catchmentson the southern flank of the volcano, the Akagawa River (~ 4.4 km2) and the Shirakawa River (~ 2.9 km2) catchments, exhibited contrasting
sediment delivery patterns and river water characteristics such as acidity and electric conductivity (EC). Increased turbidity, a high rate ofsuspended sediment supply, and elevated EC values were observed only in the Akagawa River, which hosts volcanic vents in its proximal part. The
mineral assemblages and chemical characteristics of suspended sediment from the Akagawa River clearly indicate that the turbidity was derived
from the erosion and reworking of primary eruptive material and lahar deposits. Previous airborne and remote surveys suggested the presence ofprimary ashfall and pyroclastic density currents in the upslope areas and valley heads of both the Akagawa(c)and Shirakawa rivers. However, the river water characteristics and sediment transportation data of the present
study clarify that the initial volcanic disturbance of the Shirakawa catchment was minor and limited. The
influence of volcanic disturbance on the Akagawa River catchment continued for at least 10 months after the
eruption and was also observed for an additional 9 months until the end of the snowmelt season in 2016. In
the Akagawa River valley, two post-eruptive lahars that occurred during a 7-month period may have enhanced
the removal of volcaniclastic deposits, and this remobilization may have resulted in diminished sedimentdelivery in the river after the lahar events. The results of this study provide information about the timing of thedecline of suspended sediment delivery associated with small-scale eruptive activity, and such information
may prove useful for evaluating the effects of other eruptions similar in size and character to the 2014 Ontake
eruption. In addition, the approach adopted for monitoring rivers at downstream sites is clearly of utility forevaluating primary pyroclastic deposition and volcanic disturbance near inaccessible vent areas.

Keywords: Ontake Volcano, 2014 eruption, Phreatic eruption, Suspended sediment supply, Volcanic
disturbance, Water characteristics*Corresponding author: Kyoko S. Kataoka, kataoka@gs.niigata-u.ac.jp
16

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Contents
Special issue “Towards forecasting phreatic eruptions: Examples from Hakone volcano and some global equivalents”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kazutaka Mannen, Diana Roman, Graham Leonard, Stephanie Prejean and Mitsuhiro Nakagawa
Understanding and forecasting phreatic eruptions driven by magmatic degassing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . John Stix and J. Maarten de Moor
Pyroclastic density currents associated with the 2015 phreatomagmatic eruption of the Kuchinoerabujima volcano. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nobuo Geshi and Jun’ichi ItohAnalyzing the continuous volcanic tremors detected during the 2015 phreatic eruption of the Hakone volcano. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yohei Yukutake, Ryou Honda, Masatake Harada, Ryosuke Doke,
Tatsuhiko Saito, Tomotake Ueno, Shin’ichi Sakai and Yuichi Morita

1
7
7
8

Phreatic eruptions and deformation of Ioto Island (Iwo-jima), Japan, triggered by deep magma injection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hideki Ueda, Masashi Nagai and Toshikazu Tanada 8
InSAR analysis for detecting the route of hydrothermal fluid to the surface during the 2015 phreatic eruption of Hakone
Volcano, Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ryosuke Doke, Masatake Harada, Kazutaka Mannen, Kazuhiro Itadera and Jun Takenaka 9Chronology of the 2015 eruption of Hakone volcano, Japan: geological background, mechanism of volcanic unrest anddisaster mitigation measures during the crisis. . . . . . . . . . . . . . . . . . . . . . . . Kazutaka Mannen, Yohei Yukutake, George Kikugawa, Masatake Harada, Kazuhiro Itadera and Jun Takenaka 9Contention between supply of hydrothermal fluid and conduit obstruction: inferences from numerical simulations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ryo Tanaka, Takeshi Hashimoto, Nobuo Matsushima and Tsuneo Ishido 10
Lahar characteristics as a function of triggering mechanism at a seasonally snow-clad volcano: contrasting lahars following
the 2014 phreatic eruption of Ontake Volcano, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kyoko S. Kataoka, Takane Matsumoto, Takeshi Saito, Katsuhisa Kawashima, Yoshitaka Nagahashi,
Tsutomu Iyobe, Akihiko Sasaki and Keisuke Suzuki

10Precursory tilt changes associated with a phreatic eruption of the Hakone volcano and the corresponding source model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ryou Honda, Yohei Yukutake, Yuichi Morita, Shin’ichi Sakai, Kazuhiro Itadera and Kazuya Kokubo 11
Anatomy of phreatic eruptions . . . . . . . . . . . . . . . Corentin Caudron, Benoit Taisne, Jurgen Neuberg, Arthur D. Jolly, Bruce Christenson,
Thomas Lecocq, Suparjan, Devy Syahbana and Gede Suantika

11

Relating gas ascent to eruption triggering for the April 27, 2016, White Island (Whakaari), New Zealand eruption
sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arthur Jolly, Ivan Lokmer, Bruce Christenson and Johannes Thun 12
Geophysical examination of the 27 April 2016 Whakaari/White Island, New Zealand, eruption and its implications forvent physiognomies and eruptive dynamics . . . . . . . . . . . . . . . . . . . . . Braden Walsh, Jonathan Procter, Ivan Lokmer, Johannes Thun,
Tony Hurst, Bruce Christenson and Arthur Jolly

Phreatic eruption dynamics derived from deposit analysis: a case study from a small, phreatic eruption from
Whakāri/White Island, New Zealand. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geoff Kilgour, Stephanie Gates, Ben Kennedy, Aaron Farquhar,
Ame McSporran and Cameron Asher

Time variations in the chemical and isotopic composition of fumarolic gases at Hakone volcano, Honshu Island, Japan,
over the earthquake swarm and eruption in 2015, interpreted by magma sealing model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takeshi Ohba, Muga Yaguchi, Kana Nishino, Nozomi Numanami, Yasushi Daita,
Chiho Sukigara, Masanori Ito and Urumu Tsunogai

12

13

13

Infrasonic wave accompanying a crack opening during the 2015 Hakone eruption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yohei Yukutake, Mie Ichihara and Ryou Honda 14
Resistivity characterisation of Hakone volcano, Central Japan, by three-dimensional magnetotelluric inversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ryokei Yoshimura, Yasuo Ogawa, Yohei Yukutake, Wataru Kanda, Shogo Komori, Hideaki Hase,
Tada-nori Goto, Ryou Honda, Masatake Harada, Tomoya Yamazaki, Masato Kamo, Shingo Kawasaki,
Tetsuya Higa, Takeshi Suzuki, Yojiro Yasuda, Masanori Tani and Yoshiya Usui

Temporal changes in inflation sources during the 2015 unrest and eruption of Hakone volcano, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Masatake Harada, Ryosuke Doke, Kazutaka Mannen, Kazuhiro Itadera and Mikio Satomura
Monitoring ground deformation of eruption center by ground-based interferometric synthetic aperture radar (GB-InSAR):a case study during the 2015 phreatic eruption of Hakone volcano. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Senro Kuraoka, Yuichi Nakashima, Ryosuke Doke and Kazutaka Mannen
Experimental approach to rootless eruptions using kitchen materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rina Noguchi, Ai Hamada, Ayako I. Suzuki and Kei Kurita
Suspended sediment transport diversity in river catchments following the 2014 phreatic eruption at Ontake Volcano, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kyoko S. Kataoka, Takane Matsumoto, Takeshi Saito, Yoshitaka Nagahashi and Tsutomu Iyobe

14
15

15
16
16

Cover image: Stix, J. et al., Earth, Planets and Space 2018, 70:83 DOI:10.1186/s40623-018-0855-z